Re: Re[2]: [Haskell-cafe] nested maybes
On 2/5/07, Bulat Ziganshin [EMAIL PROTECTED] wrote: Hello J., Sunday, February 4, 2007, 11:46:57 PM, you wrote: exists s wmap = isJust $ find (==s) . snd = Map.lookup (sort s) wmap exists s wmap = Map.lookup (sort s) wmap == snd == find (==s) == isJust a==b = a=return.b Very nice! Didn't know about ==. Thanks to everyone else who responded too; I'm learning a lot from this thread. martin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
Martin DeMello wrote: On 2/5/07, Bulat Ziganshin [EMAIL PROTECTED] wrote: Hello J., Sunday, February 4, 2007, 11:46:57 PM, you wrote: exists s wmap = isJust $ find (==s) . snd = Map.lookup (sort s) wmap exists s wmap = Map.lookup (sort s) wmap == snd == find (==s) == isJust a==b = a=return.b Very nice! Didn't know about ==. Thanks to everyone else who responded too; I'm learning a lot from this thread. (==) is user-defined; that's what the last line is for :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] nested maybes
I have a Data.Map.Map String - (Layout, [String]) as follows: type Anagrams = [String] type Cell = (Layout, Anagrams) type WordMap = Map.Map String Cell exists str wmap = let a = Map.lookup (sort str) wmap in case a of Nothing - False Just x - case (find (== str) (snd x)) of Nothing - False _ - True the existence test looks ugly - any more compact way to write it? martin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
On 2/4/07, Martin DeMello [EMAIL PROTECTED] wrote: I have a Data.Map.Map String - (Layout, [String]) as follows: type Anagrams = [String] type Cell = (Layout, Anagrams) type WordMap = Map.Map String Cell exists str wmap = let a = Map.lookup (sort str) wmap in case a of Nothing - False Just x - case (find (== str) (snd x)) of Nothing - False _ - True the existence test looks ugly - any more compact way to write it? How about: exists str = fromMaybe False . fmap (elem str.snd) . Map.lookup (sort str) -- Cheers, Lemmih ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
Maybe has a Monad instance, so you can write this as follows (untested): exists str wmap = boolFromMaybe exists' where exists' = do x - Map.lookup (sort str) wmap find (== str) (snd x) boolFromMaybe (Just _) = True boolFromMaybe Nothing = False /g On 2/4/07, Martin DeMello [EMAIL PROTECTED] wrote: I have a Data.Map.Map String - (Layout, [String]) as follows: type Anagrams = [String] type Cell = (Layout, Anagrams) type WordMap = Map.Map String Cell exists str wmap = let a = Map.lookup (sort str) wmap in case a of Nothing - False Just x - case (find (== str) (snd x)) of Nothing - False _ - True the existence test looks ugly - any more compact way to write it? martin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- It is myself I have never met, whose face is pasted on the underside of my mind. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
On Sun, 2007-02-04 at 19:54 +0530, Martin DeMello wrote: I have a Data.Map.Map String - (Layout, [String]) as follows: type Anagrams = [String] type Cell = (Layout, Anagrams) type WordMap = Map.Map String Cell exists str wmap = let a = Map.lookup (sort str) wmap in case a of Nothing - False Just x - case (find (== str) (snd x)) of Nothing - False _ - True the existence test looks ugly - any more compact way to write it? Using the Maybe Monad is one solution i think (as in: i _think_ this should work): findIt str wmap = do a - Map.lookup (sort str) wmap return $ find (== str) (snd a) exists str wmap = case findIt str wmap of Nothing - False Just _ - True The Maybe monad is very nice for abstracting away all those case-expressions. signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
On 2/4/07, Udo Stenzel [EMAIL PROTECTED] wrote: J. Garrett Morris wrote: Small improvement (Data.Maybe is underappreciated): exists str wmap = isJust exists' where exists' = do x - Map.lookup (sort str) wmap find (== str) (snd x) This is true. Some time ago I swore off the use of fromRight and fromLeft in favor of maybe, and have been forgetting about the other functions in Data.Maybe ever since. and maybe another improvement, though this is dependent on your tastes: exists s wmap = isJust $ Map.lookup (sort s) wmap = find (== s) . snd If you're going to write it all on one line, I prefer to keep things going the same direction: exists s wmap = isJust $ find (==s) . snd = Map.lookup (sort s) wmap Normally, from there I would be tempted to look for a points-free implementation, but in this case I have a strong suspicion that would simply be unreadable. /g -- It is myself I have never met, whose face is pasted on the underside of my mind. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
Hi This is true. Some time ago I swore off the use of fromRight and fromLeft in favor of maybe, and have been forgetting about the other functions in Data.Maybe ever since. I think you mean you swore off fromJust. Unfortunately when people started to debate adding fromLeft and fromRight they decided against logic and consistency, and chose not to add them... Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
J. Garrett Morris wrote: On 2/4/07, Udo Stenzel [EMAIL PROTECTED] wrote: exists s wmap = isJust $ Map.lookup (sort s) wmap = find (== s) . snd If you're going to write it all on one line, I prefer to keep things going the same direction: Hey, doing it this way saved me a full two keystrokes!!!1 Sure, you're right, everything flowing in the same direction is usually nicer, and in central Europe, that order is from the left to the right. What a shame that the Haskell gods chose to give the arguments to (.) and ($) the wrong order! exists s wmap = isJust $ find (==s) . snd = Map.lookup (sort s) wmap Normally, from there I would be tempted to look for a points-free implementation, but in this case I have a strong suspicion that would simply be unreadable. Well, depends on whether we are allowed to define new combinators. I sometimes use -- Kleisli composition infixl 1 @@ (@@) :: Monad m = (a - m b) - (b - m c) - (a - m c) f @@ g = join . liftM g . f and the resulting exists s = Map.lookup (sort s) @@ find (== s) . snd isJust isn't all that bad. (To be read as: one can get used to it.) I also think, (@@) and () belong in the Prelude and () at type ((a-b) - (b-c) - (b-c)) should be known under a shorter name. Unfortunately, everything short but (?) is already taken... Of course, the remaining variable s could also be transformed away, but that's really pointless. -Udo -- Never confuse motion with action. -- Ernest Hemingway signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
u.stenzel: J. Garrett Morris wrote: On 2/4/07, Udo Stenzel [EMAIL PROTECTED] wrote: exists s wmap = isJust $ Map.lookup (sort s) wmap = find (== s) . snd If you're going to write it all on one line, I prefer to keep things going the same direction: Hey, doing it this way saved me a full two keystrokes!!!1 Sure, you're right, everything flowing in the same direction is usually nicer, and in central Europe, that order is from the left to the right. What a shame that the Haskell gods chose to give the arguments to (.) and ($) the wrong order! exists s wmap = isJust $ find (==s) . snd = Map.lookup (sort s) wmap Normally, from there I would be tempted to look for a points-free implementation, but in this case I have a strong suspicion that would simply be unreadable. Well, depends on whether we are allowed to define new combinators. I sometimes use -- Kleisli composition infixl 1 @@ (@@) :: Monad m = (a - m b) - (b - m c) - (a - m c) f @@ g = join . liftM g . f By the way, this is now in Control.Monad (in darcs). Though since we also want the flipped version, it becomes: -- | Left-to-right Kleisli composition of monads. (=) :: Monad m = (a - m b) - (b - m c) - (a - m c) f = g = \x - f x = g -- | Right-to-left Kleisli composition of monads. '(=)', with the arguments flipped (=) :: Monad m = (b - m c) - (a - m b) - (a - m c) (=) = flip (=) Cheers, Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] nested maybes
On 2/4/07, Udo Stenzel [EMAIL PROTECTED] wrote: J. Garrett Morris wrote: On 2/4/07, Udo Stenzel [EMAIL PROTECTED] wrote: Well, depends on whether we are allowed to define new combinators. I sometimes use -- Kleisli composition infixl 1 @@ (@@) :: Monad m = (a - m b) - (b - m c) - (a - m c) f @@ g = join . liftM g . f I was responding to this, but Dons beat me to it. Personally, I use this combinator quite a bit. (As much as I would rather use , the Kleisli arrow is a bit verbose to use for my taste.) and the resulting exists s = Map.lookup (sort s) @@ find (== s) . snd isJust isn't all that bad. (To be read as: one can get used to it.) I also think, (@@) and () belong in the Prelude and () at type ((a-b) - (b-c) - (b-c)) should be known under a shorter name. Unfortunately, everything short but (?) is already taken... Presumably you mean (a - b) - (b - c) - (a - c)? I would personally be fine with Arrows being in the prelude (and, for instance, (.) defined as flip ()). I'd support your shorter name idea if I could think of one... /g -- It is myself I have never met, whose face is pasted on the underside of my mind. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
